1. Technical Field
The present invention relates to a toner for electrostatic image development which is used for in image formation by an electrophotographic system (hereinafter, also simply referred to as a toner).
2. Description of Related Art
Recently, in a production printing field, further increased speed, expanded types of papers and improved image quality of an image forming device such as a copying machine or a printer, have been under progress. As for the toner, it is required to attain further improvement of low-temperature fixability and high-temperature off-set resistance in order to respond to the increased speed and expanded types of papers.
To lower a fixing temperature of a toner, it is necessary to lower a melting temperature or melting viscosity of a binder resin. However, when a glass transition point or a molecular weight of a binder resin is lowered to lower the melting temperature or melting viscosity of binder resin, a new problem such as decreased high-temperature off-set resistance or lowered heat resistant storability of a toner occurs. Under the circumstances, as a technique to have the low-temperature fixability as well as the high-temperature off-set resistance and heat resistant storability, a toner with a core-shell structure having a polyester resin as a shell layer has been proposed (for example, see Japanese Patent Application Laid-Open No. 2005-338548). Specifically, the polyester resin has an advantage in that a low softening point can be easily designed while maintaining a high glass transition point, compared to a styrene-acrylic copolymer resin. Consequently, by using a polyester resin in a shell layer, a toner with excellent high-temperature off-set resistance, heat resistant storability and good low-temperature fixability can be obtained. In particular, for manufacturing a toner by emulsion aggregation process, there is an advantage in that such morphology control can be easily performed.
Meanwhile, in recent years, since types of papers have been further extended, it is required to have fixing on a transfer medium which has been remained difficult to handle like thick paper, an envelope, or a rough paper with high surface irregularity. Under the circumstances, even the toner with a core-shell structure was not found to be sufficient for satisfying the low-temperature fixability and high-temperature off-set resistance.
Furthermore, for satisfying image quality such as high color reproducibility or fine line reproducibility that have been required in the production printing field, it is effective to reduce an amount of toner transferred to a paper. To obtain a suitable image density while reducing an amount of toner transferred to a paper, it is required to fill a large amount of a color material in a toner and dispersing therein homogeneously. In particular, when carbon black generally used as a colorant of a black developer is used, carbon black cannot be satisfactorily dispersed in a resin which is adjusted to have a low molecular weight for low temperature fixing. As a result, problems have been caused that a shielding power is lowered so that sufficient image density is not obtained, and charging property of the toner is impaired so that image defects like uneven image density occur. To solve those problems, a technique of having both high glossiness and high image quality by defining a surface carboxylic acid amount of a resin constituting a toner and using carbon black with pH adjusted to 7 to 10 has been disclosed in Japanese Patent Application Laid-Open No. 2010-191343, for example.